The present invention is directed generally to an optical scanning printer of the type employing a modulated beam of light which is scanned across image recording material, as the latter is moved transversely to the direction of scanning and, in particular, to an apparatus for controlling the modulation of the beam as a function of beam position.
A number of known electronic printing systems of the foregoing kind employing modulated light beams exist. For accurately placing a pixel of light on the image recording material, the light emitting source must be turned on at the exact moment that the scanning beam would be where the pixel is to be written.
Precision of beam movement from pixel spot to pixel spot is, of course, required for satisfactory results. Precision becomes critical when photographic quality images are to be printed. With photographic quality prints, however, each pixel size is very small, for example 0.127 mm in diameter. Such size is, of course, necessary for obtaining the desired resolution needed for photographic prints. In photographic quality situations, misplacement of a relatively small number of pixels or even a single scanning line can lead to undesirable prints.
Unfortunately, misplacement of pixels is rather difficult to avoid on a consistent basis especially given the numerous potential problems that exist in the mechanics and optics of known electronic printing systems.
Correct pixel placement in some known printing devices is achieved by using a reference or tracking beam travelling in synchronization with the writing beam for feedback control of the energization of the writing beam. The tracking beam is directed to an encoding device which then reflects modulated pulses of the tracking beam to a detector. The modulated tracking beam is representative of writing beam position. The pulses when detected by the detector control timing of the energization or modulation of the writing beam. Representative examples of such approaches are disclosed in U.S. Pat. Nos.: 4,523,093; 4,600,837 and 4,616,132.
However, when printing photographic quality images, each pixel spot is small as noted above. As a result, the response time of the encoding device to correct for pixel deviation for feedback control purposes must be correspondingly faster. For instance, if the writing beam is energized for a period of 17 microseconds, it is desirable to have a feedback response time within a microsecond time frame.
It is, therefore, critical to provide a system which can quickly and accurately respond to the position of a scanning writing beam in order to compensate for writing beam deviations in placing pixels on a recording medium, especially in situations wherein photographic quality images are to be printed.